Optical high speed data signals are typically generated by modulating light of a continuous wave (CW) laser using a modulator such as a Mach Zehnder modulator (MZM) rather than directly modulating the laser bias current. The resulting non-return to zero (NRZ) signal is optionally shaped to a return to zero (RZ) signal by use of a second MZM. The bias points of the MZMs and their phase relations need to be dynamically controlled to compensate for temperature, aging and device tolerance.
The established control mechanisms for NRZ bias, RZ bias and RZ phase differ, but the mechanisms are all based on modulating a bias point with a pilot tone. The modulation results in the average optical output power being modulated with the pilot tone. Non-optimal selection of the bias point results in higher power variation of the output signal. The power variation is filtered, measured and demodulated with a synchronous rectifier to be used as feedback signal for control of the bias point. The synchronous rectifier works best if its signals are in phase. Utilizing a non-optimally phased signal to the synchronous rectifier leads to a decreased feedback gain and in turn to a less accurate bias point control. The use of this non-optimal bias point for biasing the modulators results in decreased system performance and loss of transmitted information.
Conventionally, compensation is done by a fixed phase adjustment. The phase deviation is calculated at the design phase or measured at the testing phase and then used as a fixed input signal phase offset to the synchronous rectifier. The fixed phase adjustment has many disadvantages. Additional time and effort are required at the testing phase of the system. No compensation is provided for environmental changes (i.e., temperature, component aging) of the system. Moreover, no compensation is available for frequency dependent phase deviation caused by tolerance of pilot tone.